Port multipliers allow many storage devices to connect to a single port of a host device. One port of a host device may be expanded to support a maximum of 15 connections, according to the SATA (Serial Advanced Technology Attachment) standard. Reference to the SATA standard may refer to the SATA Revision 3.0 specification, published May 29, 2009, and predecessors or derivatives thereof. According to the SATA Port Multiplier Specification, only a single level of expansion is allowed, and port multipliers cannot be connected to higher-level port multipliers. Thus, it is not possible to implement a general hub-style system with port multipliers according to the standard by deploying lower-level devices. Thus, all port multiplier expansion is assumed to occur through a single port multiplier device on a single level of expansion, without the possibility of expansion once the root port(s) are occupied.
Additionally, traditional port multipliers have a fixed identity, and are allocated a fixed amount of device ports, which fixes the amount of storage space based on their identity (as stated before no expansion is possible). For example, traditionally a 1 to 3 port multiplier is always a 1 to 3 port expander, and will always occupy no more and no fewer than 3 ports of the host device when connected. And the host port which has the 1:3 device would then be stopped from any additional expansion.
Although the SATA Port Multiplier Specification allows up to 15 connections, various practical constraints prevent utilizing more than a fraction of the available expansion connections. The cost of manufacturing of port multiplier devices increases as more ports are included. Perhaps more significantly is the performance cost, given that a single port multiplier will still have only the same amount of bandwidth (e.g., 3 Gbit/s or 6 Gbit/s) to connect to the host. Thus, practical constraints generally keep port expansion limited to port multipliers that have either 1 to 2, 1 to 4, 1 to 5, or 1 to 8 devices. As a result, a host device may be able to support more storage than can be achieved in a practical implementation.